Controlling the tightening of screw fasteners can be implemented by various methods, in particular by measuring torque, angle, or tightening force. The method in most widespread use is tightening to a particular torque, either by using wrenches that release at that torque, or by using electronic wrenches. Tightening to a particular angle is in very widespread use in the automobile industry. This can be implemented with a manual wrench including means for measuring tightening angle. Tightening to a particular force has been used in the past only on very specific connections. For example, there exist wrenches that enable force tightening to be implemented, but only up to the elastic limit. Other known wrenches allow force tightening to be performed over a broader range, but they then require tightening to be performed in three successive stages (i.e. tightening up to an estimate of the intended tightening force, followed by loosening fully, and finally tightening back up to the calculated value), and that can be harmful to the integrity of the connection. In such applications, tightening is controlled by means of ultrasound measuring systems or hydraulic tensioners.
Tightening to a particular torque has the advantage of being simple to use. In contrast, it suffers from a major drawback: for a given tightening torque, the force on the screw fastener varies considerably, due to the large dispersion in coefficient of friction. This is illustrated diagrammatically in FIG. 9. In this figure, it can be seen that for a given setpoint torque, Capplied, the result of dispersion in the apparent coefficient of friction [fmin, fmax] is dispersion [Fmin, Fmax] in the traction force F on the screw fastener, and consequently dispersion in mechanical deformation.
When lubrication is based on Teflon® for example, as is used in cryogenic engines, experience shows that dispersion of the order of 300% on the coefficient of friction need to be taken into account when dimensioning screw connections. The magnitude of this dispersion lies behind numerous difficulties, and indeed impossibilities, in specifying a setpoint torque. When specifying and tightening to a particular torque, account needs to be taken of the extreme bounds on the range over which the coefficient of friction can vary: low coefficients of friction determine the mechanical strength of the assembly, whereas higher coefficients of friction are responsible for the quality of tightening in connections (gaskets sufficiently compressed, flanges sufficiently clamped, etc.). Such a situation is not satisfactory since it leads to connections being overdimensioned, which is harmful both in terms of mass, and concerning the mechanical behavior of the fastener over time (fatigue, loosening, . . . ).
Furthermore, it is necessary to take account of the mechanical deformation of the fastener that results from the traction force which is applied thereto. During a tightening operation, deformation initially occurs in the elastic range (i.e. reversibly), with deformation varying linearly with force, after which, if tightening is continued, deformation takes place in the plastic range (i.e. irreversibly), with deformation varying progressively more quickly with increasing stress, ending in rupture. Because of this behavior, when tightening to torque, the resulting traction forces are highly dispersed, and tightening should preferably be performed in the elastic deformation range of the fasteners, keeping well away from the elastic limit.
At present, tightening wrenches exist that enable either tightening torque to be controlled on its own, as described in U.S. Pat. No. 3,710,874, or else enabling tightening torque and angle of rotation to be controlled simultaneously so as to tighten the fastener by an amount that corresponds to the intended value for tightening toque and/or for angle of rotation. Such a device is described in particular in European patent application EP 1 022 097. Finally, there exist torque wrench devices with elaborate processor means enabling the accuracy of tightening to be increased. Such a device is described in particular in French patent application FR 2 780 785. Nevertheless, with that type of device, tightening can be controlled to reach a desired value only by making use of a specific tightening procedure that includes intermediate tightening and loosening steps.
To sum up, none of the known tightening devices constitutes a manual wrench enabling the instantaneous traction force exerted on the screw fastener to be determined, even though that is the parameter which determines the quality of tightening and how it will behave over time. Furthermore, devices do not exist that enable tightening that has been interrupted to be restarted directly and without risk.